The dorsolateral pons and medial medulla have been implicated in the loss of muscle tone (atonia) of REM sleep and cataplexy. However, the synaptic organization of this ponto-medullary system is largely unknown. Indeed the anatomical localization of the medullary neurons involved in this circuit is unclear. Furthermore, while cholinergic mechanisms in the pons have been implicated in the triggering of atonia, there has been no study showing elicitation of atonia by chemical stimulation of the medulla. Thus, the transmitters activating these neurons are also unknown. We propose to identify transmitter agonists and antagonists capable of triggering or blocking atonia when microinjected at the medullary level, and map the effective sites. We have strong pilot data demonstrating that the medulla contains at least two pharmacologically and anatomically distinct regions which mediate atonia. We propose to use anatomical and electrophysiological techniques to identify the interconnections between pontine and medullary "inhibitory" regions. Using extracellular and intracellular recording techniques, we propose to identify medullary cell populations selectively active during atonia and determine the inputs, outputs and morphology of these cells. We have recently found that variables linked to blood pressure have a key role in the regulation of atonia in both the decerebrate and narcoleptic animal. We have hypothesized that central chemoreceptor activity is critically involved in the triggering of atonia. Accordingly, we will investigate the relation of circulatory variables and central chemoreceptors to the activity of the pontomedullary atonia generating system. These studies will take advantage of the rapid progress that can be made in the decerebrate preparation, to analyze the ponto- medullary inhibitory circuit. They should provide important basic information on the mechanisms controlling muscle tone, and lead to a better understanding of the generation of atonia in normal REM sleep and in cataplexy.